Modern magnetic resonance devices have several component parts which generate oscillations, particularly in the audible range. The most obvious examples of this which can be cited are the gradient coils and the coldhead. These oscillations are transmitted via acoustically hard joints between the individual component parts in a magnetic resonance device, ultimately in particular also to the cladding, so that a patient positioned in the patient receiver opening can be exposed to a high noise level.
The methods used for reducing this noise in the case of magnetic resonance devices include so-called vacuum claddings, with which the magnet, a term which in the context of this invention is to be understood as the casing which surrounds at least the superconducting elements, is encapsulated in a cladding, with the space between the cladding and the magnet being evacuated. By this means, the transmission path for structure-borne sound, between the vibrating parts of the device such as for example the gradient coils affixed to the magnets and the cladding, is cut off thus reducing the noise emitted to the patient. Nevertheless, the noise emission is not reduced completely by such a vacuum cladding, because the major factor which is then still relevant in determining the noise transmission is the attachment of the cladding. The vibrations of the oscillating magnets can propagate via the attachment points along the cladding, as a result of which high noise emissions can occur in the region of the patient opening.
In order to get round these problems of the transmission path for structure-borne sound with a vacuum cladding of this type, it has been proposed that use be made of a so-called composite material for the vacuum cladding. Such a composite material consists of GRP layers to increase the rigidity and fiber damping materials to increase the acoustic damping. It is a disadvantage that the manufacturing costs for such a cladding are extremely high due to the expensive method of manufacture, and that the rigidity of the cladding is reduced by the damping layers, which can result in mechanical deformations due to the high vacuum forces.
It has also been proposed that a reduction in the sound transmission be achieved by a suitable choice of the attachment point for the cladding, at a point on the magnet where the vibrations are as low as possible. However, such low-vibration points can only be found for a few discrete frequencies. But the oscillations which arise from the component parts which generate oscillations lie across a wide spectrum of frequencies.
Finally, it is also known that the joint of the vacuum cladding to the magnet is effected by an acoustically hard seal, which in turn permits a relevant structure-borne sound transmission.
U.S. Pat. No. 6,549,010 B2 describes a magnetic resonance device with which vibrations of the gradient coils relative to the cryostatic shell of the magnet can be damped by an attachment which is fitted with active damping elements. Another variant of an actively damped joint between the cryostatic shell and the gradient coils is described by U.S. Pat. No. 6,894,498 B2, which describes a support with a flexible damping element and an active one, in which the gradient coils are practically suspended.
DE 199 40 551 C1 describes a magnetic resonance device in which the gradient coils are arranged on an external cladding of the magnet, where they can be damped by active oscillation-damping elements. Alternatively, a coldhead can also be arranged on the external cladding, and be damped.